from the link provided earlier, "What the UK drs. say"

Reasons for low testing rates and hence low detection rates include:[8]
•Concerns about confidentiality, legal and insurance issues.
•Self perceptions of low risk in those who would test positive.
•Denial.
•Dislike of counselling.
•Wishing to avoid anxiety when waiting for results.

Fear and denial are the most common obstacles to HIV testing among those acknowledging that they have been at risk.

However, the most common reason is lack of time for pre-test counselling, even in GUM clinics.

(Stigma not even mentioned)